Cross-point memory devices, electronic systems including cross-point memory devices and methods of accessing a plurality of memory cells in a cross-point memory array

ABSTRACT

Memory devices comprise a plurality of memory cells, each memory cell including a memory element and a selection device. A plurality of first (e.g., row) address lines can be adjacent (e.g., under) a first side of at least some cells of the plurality. A plurality of second (e.g., column) address lines extend across the plurality of row address lines, each column address line being adjacent (e.g., over) a second, opposing side of at least some of the cells. Control circuitry can be configured to selectively apply a read voltage or a write voltage substantially simultaneously to the address lines. Systems including such memory devices and methods of accessing a plurality of cells at least substantially simultaneously are also disclosed.

TECHNICAL FIELD

Embodiments of the present invention relate generally to non-volatile memory. More particularly, embodiments of the present invention are directed to methods of reading and writing memory devices as well as to such memory devices.

BACKGROUND

Cross-point memories are memories that use intersecting address lines, such as row and column lines, with an intervening memory element. Examples of cross-point memories include magnetoresistive random access memory (MRAM), resistive random access memory (RRAM), ferroelectric random access memory (FRAM), silicon oxide nitride oxide semiconductor memory, polymer memory, and phase-change memory.

The memory element for conventional cross-point memories may be programmed by varying the voltage across the memory element (e.g., phase-change memory), or by varying the polarity of the voltage across the memory element (e.g., MRAM and some RRAM).

RRAM devices use resistive switching memory elements as an electronic memory. One type of RRAM memory element utilizes a material that may be, in one application, electrically switched between a first resistive value and a second resistive value based on the polarity of a current through the memory element. For example, a current pulsed though the memory element in a first polarity may cause the memory element to comprise a first resistive value representing a 1, while a current pulsed through the same memory element in a second, opposite polarity may cause the memory element to comprise a second resistive value representing a 0.

MRAM devices store information as an orientation of a magnetization. One type of MRAM memory element utilizes a magnetic tunnel junction (MJT). An MJT is typically formed from two ferromagnetic plates, each of which is configured to hold a magnetic field, separated by a thin insulating layer. One of the two plates is typically a permanent magnet set to a particular polarity, and the other plate is typically configured with a magnetic field that will change polarity to match that of an external field. A memory element may be written by passing a current through wires placed just above and below the MTJ element. A current in one polarity results in a magnetic field of the other plate in a particular polarity, while a current in the opposite polarity results in a magnetic field of the other plate that is opposite to the particular polarity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a memory device according to some embodiments.

FIGS. 2A-2D illustrate a schematic depiction of at least one embodiment of at least a portion of a single plane in a memory array.

FIGS. 3A and 3B are a schematic depiction of at least one embodiment of a portion of a single plane in a memory array.

FIG. 4 is a schematic of a computing system diagram showing at least one semiconductor memory device containing at least one cross-point memory device according to at least one embodiment of the present invention.

FIG. 5 is a truth table showing the various possible voltages to a memory element and respective selection device, as well as the written value according to at least one embodiment.

DETAILED DESCRIPTION

The illustrations presented herein are, in at least some instances, not actual views of any particular cross-point memory device but are merely idealized representations which are employed to describe the present invention. Additionally, elements common between figures may retain the same numerical designation.

Various embodiments of the present invention are directed toward embodiments of a method for reading and writing to a plurality of memory cells at least substantially simultaneously. Referring to FIG. 1, a cross-point memory device 10 is depicted in accordance with at least one embodiment of the present invention. The memory device 10 may include cross-point memory 100 comprising a plurality of address lines extending in a first direction and a plurality of intersecting address lines (e.g., transverse). As used herein, first address lines extend in a first direction (e.g., across the figure), and may be respectively illustrated and referred to herein as rows, or row address lines, extending across the page. Meanwhile, as used herein, second address lines extend in a second direction (into and out of the figure), across the first address lines, and may be respectively illustrated and referred to herein as columns, or column address lines. Such terminology is intended only to aid in describing embodiments of the present invention and is not intended to be limiting. For example, first (e.g., row) address lines may be second (e.g., column) address lines, and vice versa, depending on the orientation of the memory array. Of course, varying numbers of address lines, including larger numbers or smaller numbers, may be provided according to various embodiments. Examples of such devices are described in U.S. Publication No. 2006/0120136, now U.S. Pat. No. 7,359,227, issued Apr. 15, 2008, as well as U.S. Pat. No. 6,882,553, issued Apr. 19, 2005. While an embodiment of a cross-point memory implemented as an RRAM device is illustrated, any other cross-point memory may be used in other embodiments of the present invention. By way of example and not limitation, the cross-point memory may be implemented as a MRAM, FRAM, PCRAM, or other memories that are known to those of ordinary skill in the art in which the memory cells may be configured into a cross-point array.

As depicted in FIG. 1, pairs of adjacent, intersecting address lines define at least one cell between them (also referred to herein as a “memory cell”). Each cell, as shown in the magnified portion of FIG. 1, may include a memory element 110 and a selection device 120. In other words, according to one or more embodiments, a row address line (e.g., Row 3 in FIGS. 2A-2D, 3A and 3B) is adjacent (e.g., extends under) at least a portion of a first side of a cell, and a column address line (e.g., one of Col. 1B, 2B, 3B, or 4B in FIGS. 2A-3B) is adjacent (e.g., extends over) at least a portion of a second, opposing side of the cell.

The plurality of address lines are coupled to control circuitry configured to read or write the plurality of cells. For example, in the embodiment of FIG. 1, each row address line is electrically coupled to row decode/enable circuitry 130 that is configured to provide a respective voltage (including, e.g., a reference voltage, such as ground) to a plurality of row address lines in the array. Similarly, each column address line is electrically coupled to column decode/enable circuitry 140 that is configured to provide similar respective voltages to each column address line in the array. The memory device 10 further includes an address buffer and signal generation circuit 150, a write driver circuit 160, a read amplification circuit 170, a sense amplifier 180 and a data input/output buffer 190. The data input/output buffer 190 is configured to provide write data values to the write driver circuitry 160. A plurality of row address lines may be selected by the row decode/enable circuitry 130, and the write driver circuitry 160 may apply one or more reset pulses or set pulses to the plurality of column address lines selected by the column decode/enable circuitry 140. Various configurations for applying the one or more reset pulses or set pulses to the plurality of column address lines to store the plurality of data values in multiple memory cells are described in more detail below.

In at least some embodiments, the memory element 110 may comprise a resistive changing material suitable for non-volatile memory data storage. The resistive changing material may be a material having electrical properties (e.g., resistance) that may be changed through the application of energy such as, for example, heat, light, voltage potential, or electrical current. By way of example and not limitation, suitable resistive changing materials may include a phase-change or ionic conducting chalcogenide material, a binary metal oxide material, a perovskite oxide material, a colossal magnetoresistive material, or a polymer material.

By way of further example and not limitation, a phase-change chalcogenide material may comprise doped or undoped Ge₂Sb₂Te₅ or Sb₂Te₃. An ionic conducting chalcogenide material may comprise Ag-doped GeSe or GeS. A binary metal oxide material may comprise HfO_(x), Nb₂O₅, Al₂O₃, WO_(x), Ta₂O₅, TiO_(x), ZrO_(x), Cu_(x)O or Ni_(x)O. A perovskite oxide material may comprise doped or undoped SrTiO₃, SrZrO₃, or BaTiO₃. A colossal magnetoresistive material may comprise Pr_((1-x))Ca_(x)MnO₃ (PCMO), La_((1-x))Ca_(x)MnO₃ (LCMO), or Ba_((1-x))Sr_(x)TiO₃. A polymer material may comprise AlQ₃Ag, Cu-TCNQ, DDQ, TAPA, or a fluorescine-based polymer. One of ordinary skill in the art will recognize that other materials may be suitable. Thus, the scope of the present invention is not limited to just these materials.

In at least some embodiments, the selection device 120 may comprise a material that does not change phase, but remains permanently amorphous and its current-voltage characteristics may remain at least substantially the same throughout its operating life. By way of example and not limitation, the selection device 120 may comprise As—Te—I, TiAs(Se, Te)₂, Si—Te—As—Ge, Si—Te—As—Ge—P, Al—As—Te, Al—Ge—As—Te, Te₃₉As₃₆Si₁₇Ge₇P₁, although the scope of embodiments of the present invention is not limited to just these materials. In other embodiments, the selection device 120 may comprise a material that possesses bipolar rectification properties.

When an appropriate potential is applied between adjacent, intersecting address lines (e.g., Row 3 and Col. 4B in FIG. 2A), a selection device 120 between the intersecting address lines turns on, allowing current flow across a memory element 110. Thus, it may be understood that shared address lines are utilized. Namely, address lines can be shared between an overlying cell and an underlying cell. For example, with reference to FIG. 2A, column address line Col. 1A is shared between overlying cells including the memory element 110A and the selection device 120A and underlying cells including the memory element 110E and the selection device 120E. Similarly, the row address line Row 2 functions for selection of its overlying cells, including the memory elements 110E-110H and the selection devices 120E-120H, as well as for selection of its underlying cells, including the memory elements 110I-110L and the selection devices 120I-120L. As a result, considerable economies may be achieved in some embodiments of the present invention, in terms of cost, size and speed.

In one or more embodiments, a voltage biasing system is used such that address lines can be shared. In particular, a read or write voltage may be applied to one or more address lines. For example, referring to FIG. 2A, a read or write voltage may be applied to a row address line, such as Row 2. One or more columns may be selected by providing a reference voltage, such as ground, on the appropriate column address lines. For example, column address lines Col. 1A and Col. 3A, located above Row 2, and Col. 1B and Col. 2B, located below Row 2, may be grounded (e.g., by biasing those lines to 0V). Similarly, a read or write voltage may be applied to a column address line, such as column address lines Col. 4A, Col. 3B and Col. 4B, while a row address line, such as row address lines Row 1 and Row 3, are grounded. In such an embodiment, cells made up of the selection devices 120D, 120E, 120G, 120I, 120J, 120O and 120P and the memory elements 110D, 110E, 110G, 110I, 110J, 110O and 110P will be selected for reading or writing thereto.

In embodiments in which the memory element is a non-volatile, bipolar switching material (e.g., a conventional bipolar binary oxide), the memory element may be programmed into one of at least two memory states by applying an electrical signal to the memory material, the electrical signal having one of two opposing polarities referred to herein as positive or negative. A positive or negative electrical signal may change the resistance of the memory material between a relatively high resistance state and a relatively low resistance state. Programming of the memory element to change the resistance of the material may be accomplished by applying different voltages to respective crossing address lines, thereby generating a voltage potential across the memory material in a particular polarity.

The information stored in the memory elements 110 may be read by measuring the resistance of the memory material. As an example, a read current may be provided to the memory material using crossing address lines, such as Row 1 and Col. 1A, and a resulting read voltage across the memory material may be compared against a reference voltage using, for example, a sense amplifier (not shown). The read voltage may be proportional to the resistance exhibited by the memory material.

In a low-voltage or low-field regime, the selection device 120 is off and may exhibit very high resistance in some embodiments. The off resistance can, for example, range from 50,000 ohms to greater than 10 gigaohms at a bias of half the threshold voltage. The selection devices 120 may remain in the off state until a threshold voltage or threshold current switches the selection device 120 to a highly conductive, low resistance on state. The voltage across the selection device 120 after it is turned on may drop to a slightly lower voltage, called the holding voltage and may remain very close to this holding voltage. By way of example and not limitation, the threshold voltage may be on the order of about 1.1 volts and the holding voltage may be on the order of about 0.9 volt.

In the on state, the selection device 120 voltage drop remains close to the holding voltage as the current passing through the device is increased up to a certain, relatively high, current level. Above that current level the device remains on but displays a finite differential resistance with the voltage drop increasing with increasing current. The selection device 120 may remain on until the current through the selection device 120 is dropped below a characteristic holding current value that is dependent on the area of the material and may be impacted by top and bottom electrodes utilized to form the selection device 120.

In at least some embodiments, the memory elements 110 may be configured to store a 1 when a voltage across a respective memory element 110 and selection device 120 is positive voltage V_(o), and to store a 0 when the voltage across a respective memory element 110 and selection device 120 is a negative voltage V_(o). As used herein, voltage V_(o) comprises a voltage sufficient to switch the selection device 120 to the on state and to change the resistance of the memory element 110. In each of the embodiments shown in FIGS. 2A-3B, and by way of example and not limitation, the memory elements 110 and selection devices 120 are configured such that a positive voltage V_(o) alters the memory element 110 to a high resistance, while a negative voltage −V_(o) alters the memory element 110 to a low resistance. Furthermore, the embodiments described herein with reference to FIGS. 2A-3B also illustrate, by way of example and not limitation, embodiments wherein the voltage applied is considered positive when the upper conductor of a respective memory element 110 and section device 120 (as oriented in the FIGS.) is a higher voltage than the lower conductor, and negative when the lower conductor is a higher voltage than the upper conductor.

FIG. 5 is a truth table showing the various possible voltages to a memory element 110 and respective selection device 120, as well as the resulting change, if any, to the memory element 110. As shown, when a voltage of 0V is applied to both the upper conductor (referred to as “top” in FIG. 5) and the lower conductor (referred to as “bottom” in FIG. 5), the voltage from top to bottom is 0V and nothing is written to the respective memory element 110. When the voltage at the upper conductor is 0V and the voltage at the lower conductor is ½V_(o), the voltage from top to bottom is −½ V_(o) and nothing is written to the respective memory element 110. When the voltage is 0V at the upper conductor and a voltage V_(o) is applied to the lower conductor, the resulting voltage from top to bottom is −V_(o), resulting in a 0 being written to the respective memory element 110. When a voltage of ½V_(o) is applied to the upper conductor, nothing is written to the respective memory element 110, since a voltage of 0V on the lower conductor results in ½V_(o) from top to bottom, a voltage of ½V_(o) on the lower conductor results in 0V from top to bottom, and a voltage of V_(o) on the lower conductor results in −½V_(o) from top to bottom. When the voltage at the upper conductor is V_(o) and a voltage of 0V is applied to the lower conductor, the resulting voltage from top to bottom is V_(o), resulting in a 1 being written to the respective memory element 110. In the remaining two cases, when a voltage of V_(o) is applied to the upper conductor, a voltage of ½V_(o) on the lower conductor results in ½V_(o) from top to bottom and a voltage of V_(o) on the lower conductor results in 0V from top to bottom, both cases resulting in nothing being written to the respective memory element 110.

For example, a 1 may be written to each memory element 110I and 110J in the embodiment illustrated in FIG. 2A when the row address line Row 2 is biased at voltage V_(o) and the column address lines Col. 1B and Col. 2B are grounded. Similarly, a 1 may be written to each memory element 110O and 110P when the row address line Row 3 is grounded and the column address lines Col. 3B and Col. 4B are biased at voltage V_(o).

Furthermore, the memory elements 110 may be configured to store a 0 when a voltage across a respective memory element and selection device is −V_(o). For example, row address line Row 1 or Row 3, or both, may be grounded. One or more columns may be selected by applying a voltage V_(o) to the appropriate column address line. For example, a voltage V_(o) may be applied to the column address lines Col. 1A, Col. 3A and Col. 4A. In such an embodiment, cells made up of the selection devices 120D, 120E and 120G and the memory elements 110D, 110E and 110G will be selected with a −V_(o), resulting in a 0 being written to each memory element 110D, 110E and 110G.

At the same time, unselected address lines, such as the column address line Col. 2A, may be biased at a voltage less than the read or write voltage and greater than the reference voltage. By way of example and not limitation, unselected address lines may be biased at half the write voltage (i.e., one-half V_(o)) such that the potential across cells of the unselected address lines (e.g., selection devices 120B and 120F and memory elements 110B and 110F in FIG. 2A) is a positive or negative one-half V_(o). In this manner, the potential across unselected cells is either zero (e.g., in the case of selection devices 120A, 120C, 120H, etc., and memory elements 110A, 110C, 110H, etc., in FIG. 2A) or positive or negative one-half V_(o) (e.g., in the case of selection devices 120B and 120F and memory elements 110B and 110F in FIG. 2A). The selection devices may be configured such that the half voltage is insufficient to turn on the selection devices 120 resulting in no current flow or relatively low current flow across a cell, the memory elements 110 may be configured such that the half voltage is insufficient to change the resistance of the memory element, or a combination thereof.

This configuration allows for multiple memory elements 110 to be written to or read at least substantially simultaneously. In some embodiments, at least a quarter of a page may be written in a single clock cycle. In some embodiments, a word may be written in no more than two clock cycles. Thus, as will be seen, a reduction in clock cycles and, therefore, time for writing or reading memory elements 110 is provided. FIG. 2A is a schematic depiction of at least one embodiment of at least a portion of a single plane in a memory array. As used herein, a “single plane” in the memory array includes each of the first address lines extending parallel to each other and located in a common vertical plane of the memory array. For example, in the embodiment set forth in FIG. 2A with the row address lines Row 1 and Row 3 grounded, and a write voltage V_(o) applied to address line Row 2, one or more of column address lines Col. 1A through Col. 4B may be selected and pulsed simultaneously with voltages of 0V or V_(o) to write a 1 or 0 value to at least some of the memory elements 110, as described herein above. Thus, in the example shown in FIG. 2A, a total of 7 bits are written in a single clock cycle.

FIG. 2B illustrates the cross-point memory 100 of FIG. 2A after the memory elements 110 described above have been written. Subsequent to writing the memory elements 110 described above, it may be desired to write to memory elements 110 on the opposing side of at least some of the address lines. For example, it may be desired to write a 1 to memory element 110L, as well as to write a 0 to memory elements 110A and 110H. Thus, as shown in FIG. 2B, a voltage V_(o) may be applied to the column address line Col. 1A, and the column address lines Col. 4A and Col. 4B may be grounded. In order to protect the remaining memory elements 110, a voltage of one-half V_(o) may be applied to column address lines Col. 2A, Col. 3A, Col. 1B, Col. 2B and Col. 3B.

FIG. 2C illustrates the cross-point memory 100 of FIGS. 2A and 2B after the memory elements 110 have been written as described with reference to FIG. 2B. In FIG. 2C, subsequent to writing the memory elements 110 described above with reference to FIGS. 2A and 2B, the row address lines Row 1-Row 3 may have opposite voltages applied thereto. As illustrated, row address lines Row 1 and Row 3 may have voltage V_(o) applied thereto, while row address line Row 2 may be grounded. The voltage levels applied to column address lines Col. 1A through Col. 4B depends on the desired value written to the memory elements 110 not yet containing a value. By way of example and not limitation, a value of 1 might be desired to be written to memory elements 110B and 110C, and a value of 0 might be desired to be written to memory element 110M, 110N and 110K. In such a case, as shown in FIG. 2C, a voltage V_(o) may be applied to the column address line Col. 3B and column address lines Col. 2A, Col. 3A, Col. 1B and Col. 2B may be grounded. In order to protect the remaining memory elements 110, a voltage of one-half V_(o) may be applied to column address lines Col. 1A, Col. 4A and Col. 4B.

To complete the page illustrated in FIGS. 2A-2D, the last remaining memory element 110F may have a 1 written thereto, for example. In such a case, as illustrated in FIG. 2D, a voltage V_(o) may be applied to column address line Col. 2A while a voltage of one-half V_(o) may be applied to all the remaining column address lines Col. 1A, Col. 3A, Col. 4A, Col. 1B, Col. 2B, Col. 3B and Col. 4B in order to protect the associated memory elements 110. In such a manner, the entire page illustrated in FIGS. 2A-2D may be written to in just a few clock cycles.

In at least one other embodiment, a cross-point memory 100 may be configured to read/write a full word in only two clock cycles. By way of example and not limitation, FIG. 3A illustrates an embodiment cross-point memory 100 configured to read/write a word comprising 8 bits. As illustrated, row address line Row 2 may have a voltage V_(o) applied thereto while row address lines Row 1 and Row 3 may have a voltage one-half V_(o) applied thereto. If, for example, an eight-bit word comprises 00100110, a first plurality of data may be written at least substantially simultaneously and a second, remaining plurality of data may be written at least substantially simultaneously. By way of example and not limitation, with a voltage V_(o) applied to row address line Row 2, column address line Col. 1A, Col. 2A, Col. 4A, Col. 2B and Col. 3B may be grounded. Upon pulsing the voltage values to the column address lines, each of memory elements 110J and 110K will have a value of 1 written thereto and each of memory elements 110E, 110F and 110H will have a 0 written thereto. Each of the remaining column address lines Col. 3A, Col. 1B and Col. 4B may have either a voltage V_(o) applied thereto or a voltage one-half V_(o) applied thereto to protect their corresponding memory elements 110 from being enabled.

FIG. 3B illustrates the cross-point memory 100 of FIG. 3A subsequent to writing the first plurality of data as described above. Row address line Row 2 may be grounded while the one-half V_(o) voltage is still applied to row address lines Row 1 and Row 3. To write a value of 1 to memory element 110G and a 0 value to memory elements 110I and 110L, a voltage V_(o) is applied to column address lines Col. 3A, Col. 1B and Col. 4B. Each of the remaining column address lines Col. 1A, Col. 2A, Col. 4A, Col. 2B and Col. 3B may either be grounded or have a voltage of one-half V_(o) applied thereto to protect their corresponding memory elements 110 from being enabled.

The example provided herein with reference to FIGS. 3A and 3B describe the column address lines being pulsed to write to respective memory elements 110. However, this is not intended to be limiting. Indeed, in other embodiments, the row address lines may be pulsed after a particular voltage is applied to the respective column address lines. Thus, for example, in the embodiment described in FIGS. 3A and 3B, the respective voltages may be applied to each of the column address lines. A voltage V_(o) may be pulsed on row address line Row 2 followed by grounding the same row address line Row 2. In such an embodiment, it may not be necessary to change the voltages applied to the column address lines.

As described, an 8-bit word may be written to a plurality of memory elements 110 in just a couple of clock cycles and in close proximity in the cross-point memory array. Furthermore, as will be apparent to those of ordinary skill in the art, by providing a memory array with a greater number of cells comprising a selection device 120 and a memory element 110, a system employing larger word sizes (e.g., 16 bit, 32 bit, etc.) may similarly be written such that at least one word may be read/written in a minimum number of clock cycles.

Still additional embodiments of the present invention are directed to electronic systems comprising cross-point memory devices. As shown in FIG. 4, an electronic system 400, such as a computer system, in accordance with an embodiment of the present invention, comprises at least one input device 410, at least one output device 420, at least one memory access device, such as processor 430, and at least one memory device 440. As used herein, the term “computer system” includes not only computers, such as personal computers and servers, but also wireless communications devices (e.g., cell phones, personal digital assistants configured for text messaging and email), cameras, chip sets, set top boxes, controllers, vehicle and engine control and sensor systems, digital music players, and other combinations of the above-referenced input, output, processor and memory devices. The at least one memory device 440 comprises at least one cross-point memory device (e.g., device 100) comprising a plurality of memory cells having a memory element coupled to a selection device.

CONCLUSION

Various embodiments of the present invention are described above and comprise methods for at least substantially simultaneously accessing a plurality of cross-point memory cells. In one or more embodiments, such methods may comprise biasing a plurality of first address lines and biasing a plurality of second address lines. Each first address line of the plurality may be adjacent a first side of some of a plurality of cells in a single plane of a three-dimensional array of cells. Each cell may comprise a memory element and a selection device. The plurality of second address lines may extend across the plurality of first address lines. Each second address line may extend over a second, opposing side of at least one cell of the plurality of cells in the single plane of the three-dimensional array of cells such that a plurality of cells in the single plane of the array of cells are accessed at least substantially simultaneously and at least one cell of the plurality of cells in the single plane is not accessed.

In other embodiments, methods of storing a plurality of data values into a selected portion of memory cells in a common plane of a three-dimensional memory array may comprise biasing at least one first address line extending in first direction. At least one side of each of a plurality of cells in the common plane may be adjacent to at least one of the first address lines, each cell comprising a memory element and a selection device. A plurality of second address lines extending across the at least one first address line may be biased. Each second address line may be adjacent a side of a cell of the plurality of cells opposite the side of the cell adjacent to a respective one of the at least one first address lines. At least some of the plurality of data values may be stored at least substantially simultaneously in at least some of the plurality of cells.

In still other embodiments, methods of at least substantially simultaneously writing to a plurality of memory cells may comprise selectively applying a reference voltage to at least one first address line in a plane. Each of the at least one first address lines may be adjacent to a first side of some of a plurality of cells comprising a memory element and a selection device. A write voltage or a voltage less than the write voltage and greater than the reference voltage may be selectively applied to at least one other first address line located in the plane and adjacent to the at least one first address line. The write voltage or the reference voltage may be selectively applied to a plurality of second address lines extending across the at least one first address line. Each second address line of the plurality of second address lines may be adjacent to a second, opposing side of a respective cell of the plurality of cells.

In other embodiments a memory device may comprise a plurality of cells, each cell comprising a memory element and a selection device. A plurality of first address lines may be adjacent a first side of at least some cells of the plurality of cells. A plurality of second address lines may extend across to the plurality of first address lines, and each second address line may be adjacent a second, opposing side of at least some cells of the plurality of cells. Control circuitry may be configured to selectively apply a write voltage or a reference voltage at least substantially simultaneously to the plurality of first address lines and to selectively apply a write voltage, a reference voltage, or a voltage less than the write voltage and greater than the reference voltage at least substantially simultaneously to the plurality of second address lines.

In still other embodiments, an electronic system may comprise a memory access device and a memory device coupled thereto. The memory device may comprise a plurality of memory cells in a common plane. Each memory cell may comprise a memory element coupled to a selection device. A first address line of a plurality of first address lines may be adjacent a first side of at least some cells of the plurality of cells. A plurality of second address lines may extend across the plurality of first address lines. Each second address line may be adjacent a second, opposing side of at least some cells of the plurality of cells. A first decode/enable circuitry may be configured to selectively apply a write voltage or a reference voltage to the first address lines. A second decode/enable circuitry may be configured to selectively apply a write voltage, a reference voltage, or a voltage less than the write voltage and greater than the reference voltage to at least some of the plurality of second address lines such that at least some of the plurality of memory cells are accessed at least substantially simultaneously.

Although the embodiments described herein are directed to cross-point memories configured for bipolar switching (e.g., RRAM, MRAM), other memory may be configured for monopolar switching (e.g., PCRAM). In such embodiments, the biasing schemes described above may need to be modified slightly to protect specific cells. For example, in some monopolar switching embodiments, the value written to a memory cell may depend on the magnitude of the voltage, the length of the pulse, or both through the memory cell. Accordingly, in at least some monopolar embodiments, the memory material may be adapted to be altered such that it can be in any particular one of a number of resistance states so as to provide digital or analog storage of information. In any of the various bipolar or monopolar embodiments, it will be apparent to one of ordinary skill in the art that a plurality of cells may be simultaneously accessed by properly biasing the plurality of row address lines and the plurality of column address lines, similar to the manner described herein.

Therefore, while certain embodiments have been described and shown in the accompanying drawings, such embodiments are merely illustrative and not restrictive of the scope of the invention, and this invention is not limited to the specific constructions and arrangements shown and described, since various other additions and modifications to, and deletions from, the described embodiments will be apparent to one of ordinary skill in the art. Thus, the scope of the invention is only limited by the literal language, and legal equivalents, of the claims which follow. 

1. A memory device, comprising: a plurality of cells, each cell comprising a memory element and a selection device; a plurality of first address lines in a first plane; a plurality of second address lines in a second plane extending across the plurality of first address lines, wherein at least some cells of the plurality of cells are connected at cross-points between the first address lines and the second address lines; a plurality of third address lines in a third plane, wherein at least some cells of the plurality of cells are connected between cross-points of the second address lines and the third address lines; and control circuitry configured to selectively apply a write voltage or a reference voltage substantially simultaneously to the plurality of first address lines and third address lines, and to selectively apply a write voltage, a reference voltage, or a voltage less than the write voltage, and greater than the reference voltage, substantially simultaneously to the plurality of second address lines.
 2. The memory device of claim 1, wherein each cell of the plurality of cells comprises a bipolar memory element.
 3. The memory device of claim 2, wherein each cell of the plurality of cells comprises a monopolar switching memory element comprising a phase-change memory element.
 4. The memory device of claim 1, wherein each cell of the plurality of cells comprises a selection device comprising a material configured to retain at least substantially the same current-voltage characteristics.
 5. The memory device of claim 1, wherein the control circuitry comprises: a first decode/enable circuit coupled to the plurality of first address lines and configured to provide the write voltage or the reference voltage substantially simultaneously to the plurality of first address lines; and a second decode/enable circuit coupled to the plurality of second address lines and configured to provide the write voltage, the reference voltage or the voltage less than the write voltage and greater than the reference voltage substantially simultaneously to the plurality of second address lines.
 6. A system, comprising: a memory access device; a memory device coupled to the memory access device, the memory device comprising: a plurality of memory cells in a common plane, each memory cell comprising a memory element coupled to a selection device; a plurality of first address lines in a first plane; a plurality of second address lines in a second plane extending across the plurality of first address lines, wherein at least some cells of the plurality of cells are connected at cross-points between the first address lines and second address lines; a plurality of third address lines in a third plane, wherein at least some cells of the plurality of cells are connected between cross-points of the second address lines and the third address lines; a first decode/enable circuit configured to selectively apply a write voltage or a reference voltage to the first address lines and the third address lines; and a second decode/enable circuit configured to selectively apply a write voltage, a reference voltage, or a voltage less than the write voltage, and greater than the reference voltage, to the plurality of second address lines such that at least some cells of the plurality of memory cells are accessed at least substantially simultaneously.
 7. The system of claim 6, wherein the memory device comprises a Resistive Random Access Memory (RRAM).
 8. The system of claim 6, wherein the memory device comprises a Phase-Change Random Access Memory (PCRAM). 